The present invention relates to a method for driving a liquid crystal display device such as a liquid crystal light modulator.
In recent years, there has been growing demand for higher performance projection type displays for use as large displays for personal theaters, flat displays for personal computers and the like.
Studies have been conducted concerning liquid crystal displays (hereinafter called LCD) as a type of display device which comprehensively meets the demand. An LCD can be a low-profile, lightweight model which provides a high picture quality with low power consumption.
Currently available LCDs use either the STN (Super Twisted Nematic) birefringence mode or the TN (Twisted Nematic) mode. Furthermore, next-generation LCDs such as ferroelectric and antiferroelectric LCDs which use the birefringence mode have been studied and are expected to be commercialized in the near future. In addition to already commercialized STN displays, research in FLC (Ferroelectric Liquid Crystal) as a typical birefringence LCD has been actively conducted since the SSFLC (Surface Stabilized Ferroelectric Liquid Crystal) was proposed.
Usually in ferroelectric liquid crystals, state 1 and state 2 concerning the orientation of liquid crystal molecules M with respect to externally applied electric field E (Ps denotes spontaneous polarization) are switched in the chiral smectic (C) phase, as shown in FIG. 1. As viewed from above, the central axis of a virtual cone shown in FIG. 1 coincides with the orientation of the alignment layer (rubbing direction for rubbing films, or evaporation direction for obliquely evaporated SiO films). A change in the orientation of liquid crystal molecules M is represented as a change in light transmittance when the liquid crystal element is placed between polarizer plates which are orthogonal to each other; as shown in FIG. 2, the transmittance sharply changes from 0% to 100% at threshold Vth with respect to the impressed electric field.
SSFLC displays are fast in response (approx. 1000 times faster than conventional nematic LCDs) and have the ability of memory, which solves the problem of flickers often seen in cathode ray tubes and TN displays. Even when a simple X-Y matrix drive is used, the drive can be performed with more than 1,000 scanning lines. Because an active device such as TFT (Thin Layer Transistor) is not used, the manufacturing yield rate can be improved.
Experiments on application of ferroelectric liquid crystals for reflective displays have been carried out. Some such experiments have been disclosed in detail in IEEE Journal of Quantum Electronics, vol. 29, no. 2 (1993)699, Journal of the Society for Information Display, vol. 5 (1997)1, SPIE, vol. 3013 (1977) 174, and so on. In these experimental displays, ferroelectric liquid crystal cells are made on a semiconductor memory and the memory voltage is used for drive.
The present invention's applicant et al have already proposed a reflective display which combines a semiconductor memory and ferroelectric liquid crystal. In this display technique, gradations can be expressed by combining the field sequential process and brightness modulation of the light source; in principle, the technique can express gradations which look continuously changing tones to the human eyes.
This reflective ferroelectric LCD has ferroelectric liquid crystal 4 filled between a transparent substrate 1a and a silicon substrate (silicon VLSI circuit board) 2a, as exemplified in FIG. 3. This reflective ferroelectric LCD is made by the following process. First, a transparent electrode 1b (ITO, etc) and obliquely evaporated SiO film or high molecular thin film (typically polyimide) are formed in the inner face of a transparent substrate 1a (glass, etc) by baking, then a liquid crystal alignment layer 1c is made by rubbing, and these are laid one upon another in a given order, to make a laminate. Similarly, a reflective film/electrode 2b (ITO, etc) and obliquely evaporated SiO film or high molecular thin film (typically polyimide) are formed in the inner face of a silicon substrate 2a having a drive circuit inside each pixel) by baking, and then a liquid crystal alignment layer 2c is made by rubbing. The transparent substrate 1a and the silicon substrate 2a are arranged so that these laminates are facing each other; a granular spacer 3 is put between them to make a prescribed liquid crystal cell gap; and ferroelectric liquid crystal 4 is filled into this cell gap and the area surrounding the gap is sealed using a glue.
The pixels in the ferroelectric liquid crystal device 11 shown in FIG. 3 have a 2-dimensional structure. As shown in FIG. 4, incident light 5 to the ferroelectric liquid crystal device 11 is reflected by the reflective film/electrode 2b to exit the ferroelectric liquid crystal device as reflected light 6. The light transmittance of the ferroelectric liquid crystal 4 which lies in the optical path for the incident light 5 and the reflected light 6 varies depending on the electric field between the electrode 1b and the reflective film/electrode 2b, as shown in FIG. 2. In short, since the intensity of the reflected light 6 is modulated by the strength of the electric field between the electrode 1b and the reflective film/electrode 2b, a picture can be displayed by switching between the reflective and non-reflective states of the incident light for each pixel.
The voltage impressed on the reflective film/electrode 2b is controlled for each pixel by a control circuit 7 which is located outside the ferroelectric liquid crystal device 11. However, it may be controlled by a circuit formed on the silicon substrate 2a. Impression of voltage may be done by either scanning for each pixel or a plurality of pixels or scanning for all pixels at a time.
FIG. 5 shows a transparent liquid crystal device 21. The difference of this transparent liquid crystal device 21 from the reflective liquid crystal device shown in FIGS. 3 and 4 is that the drive electrode consists of a transparent ITO 12b on a glass substrate 12a. In this structure, the drive electrode is driven for each pixel by a control gate element 18 which consists of a TFT, and incident light 15 is transmitted as transmitted light 16 or intercepted by turning on or off the signal voltage. In a mode such as the SSFLC mode which has the effect of memory, a simple matrix drive as mentioned above, which does not use an active element, is possible.
In the TN mode, a continuous transition between the light and dark states can be made according to the effective field strength. On the other hand, it has been thought that in the SSFLC mode, it has been thought that since it features a bistability (or ability of memory) that the light transmittance (or reflectance) suddenly changes at the threshold of impressed voltage, only two states (light or dark) are selectable and middle tones between the light and dark states can be hardly controlled.
The methods for representing middle tones or gradations which have been suggested so far include: an area gradation method in which subpixels are provided and control is done according to the integrated area of the subpixels; and a multi-domain method in which microscopic inverted domains are handled by control of the amount of injected charges for each pixel. The former method has the following problems: practically a larger number of pixels are used, so the drive circuit is complicated and it is difficult to increase the resolution. The latter method has the problem that variation in temperature distribution or active element performance makes it difficult to achieve equal gradation characteristics for every pixel. Therefore, these methods cannot control gradations satisfactorily.
The present invention's applicant et al have proposed an LCD drive method in Japanese Patent Laid-Open Applications No. Hei 7-212686 and No. Hei 9-044130. In principle, this method is intended to digitally represent gradations which look continuously changing tones to the human eyes by using an on/off type spatial light modulator for reflected or transmitted light and combining the field sequential process with light source brightness modulation.
In this LCD drive method, one frame is divided into several sub-frames (defined as bit planes) and each bit plane is weighted by brightness modulation of the light source for gradation representation.
In other words, if a light source with the same light intensity is used, one frame of 16.7 msec is simply time-divided by 8 bits (0 to 256 gradation steps) to represent 8-bit gradations (256 steps). To this end, the ferroelectric liquid crystal must be completely driven in approx. 65.5 μsec. For 10-bit gradation representation, the time for driving the ferroelectric liquid crystal is 16.3 μsec. Considering the response speeds for currently available ferroelectric liquid crystal materials, it is difficult to realize this, so the impressed voltage must be increased to realize it.
As a solution, a light source whose intensity can be modulated is used to drastically lengthen the drive time for the ferroelectric liquid crystal which is determined by time-division of one frame. Here, for 8-bit gradation representation, if the light intensity of the light source can be modulated for 8 bits, it is sufficient to drive the ferroelectric liquid crystal in approx. 2.08 msec. For 10-bit gradation representation, the required drive time for the ferroelectric liquid crystal is approx. 1.67 msec. Therefore, this LCD drive method is practical since it suits the actual response speed of ferroelectric liquid crystals.
Here, a picture which consists of one gradation bit is called a “bit plane” and the time required for representing it is called a “bit plane time.” As shown in FIG. 6, if 8-bit gradation is to be represented, the number of bit planes used is 8 and the sum of eight bit plane times constitutes one frame.
It is said that in the recent digital gradation representation method used in what is called “plasma display panels,” 8-bit representation is sufficient for the minimum gradation quality but insufficient for higher picture quality.
On the other hand, digital gradation representation has a problem of false contours. This problem occurs due to a long bit plane time as a result of time division in field sequential drive: this phenomenon arises when the temporal shift of a light emitting pattern is converted into a spatial shift as the human eyes follow light emitting points. This problem can be reduced by shortening the bit plane time.
However, actually the lower limit for one bit plane time is determined by various factors such as ferroelectric liquid crystal drive response time, device structure, electric power consumption and data transmission rate. In addition to the problem of false contours, the upper limit for one bit plane time is determined by color splits, the number of gradation steps or other factors. Considering that the frame frequency is 60 Hz, usually one bit plane time should be set within the range from one hundred micron seconds to hundreds of micron seconds.
For instance, in a display device proposed by this applicant et al, 256 gradation steps are used for each of R (red), G (green) and B (blue); one frame consists of 108 bit planes (36 bit planes×3 colors); and one bit plane time is approx. 150 μsec. In this case, the ferroelectric liquid crystal is designed to be switched at least once for every bit plane.
In field sequential gradation representation, the drive voltage waveform for the ferroelectric liquid crystal must be used in one bit plane time and thus only a simple drive voltage waveform can be used. Besides, for high definition pictures, the unit pixel area is smaller and the relevant drive circuit must be built in that pixel area, which means that a simpler drive voltage waveform is required to reduce the load on the drive circuit, etc.
It is known that in the LCD manufacturing process, the liquid crystal comes to contain mixed or produced impurity ions at various steps such as liquid crystal synthesis, making of alignment layers and injection of liquid crystal, which leads to a deterioration in the quality of displayed pictures.
At present, it seems impossible to remove impurity ions in a liquid crystal panel completely. Even if they can be completely removed, impurity ions are newly generated when a voltage is impressed to drive the LC panel. Behavior of these impurity ions in the liquid crystal panel is considered as follows:    (1) A temperature rise, voltage impression or the like encourages dissociation of ions in the liquid crystal.    (2) Electrically charged ions move along the electric field in the liquid crystal generated by impression of voltage.    (3) As ions reach the alignment layer, they are adsorbed physically or chemically.    (4) If the waveform of the voltage impressed on the cell is alternating, ions are adsorbed and released repeatedly.    (5) Some of the dissociated ions return to neutral molecules by re-bonding of ions.
In this ionic behavior, if any of the following asymmetric conditions occurs in the two facing electrode substrates for drive, asymmetric ionic behavior arises in the interface between the liquid crystal and alignment layer.    (1) Structural asymmetry between two facing electrode substrates (between a TFT substrate and an ITO substrate, or between a reflective substrate and a transparent substrate in a reflective cell)    (2) Asymmetry in various conditions of the alignment layers on two facing electrode substrates (layer thickness, baking condition, rubbing strength, etc.)    (3) Asymmetry in impressed voltage waveform (in case there is waveform asymmetry for GND though an AC waveform such as a rectangular waveform is used as a general drive voltage waveform)
These asymmetric conditions cause an adsorption/release imbalance in the interface of the cationic and anionic alignment layers or an ionic polarization imbalance in the two facing electrode substrates. In this condition of ionic polarization, relaxation is difficult, so there occurs a condition similar to one which occurs when a DC component (V′) with a certain polarity is externally applied between liquid crystal cells.
This means that, even if voltage impression is stopped later, the condition that voltage V′ is impressed or that a voltage is impressed on liquid crystal molecules, is maintained inside liquid crystal cells. In other words, even when symmetric rectangular waveform voltage (amplitude V) is impressed on liquid crystal cells, the effective voltage impressed inside the liquid crystal is (V+V′) on the positive side and (−V+V′) on the negative side and thus the effective voltage impressed on the liquid crystal is no longer symmetric. In LCDs like TN mode ones in which the effective voltage is reflected in the light transmittance, this asymmetry may cause liquid crystal molecules to waver, which would be observed as a phenomenon called a “flicker,” one of the reasons for picture quality deterioration.
On the other hand, in the SSFLC mode, when a positive voltage signal (V) is used as the voltage signal for selecting one of the On state and Off state and a negative voltage signal (−V) as the voltage signal for selecting the other state, if V′ has a positive value, application of a negative voltage signal has the effect of impression of (−V−V′) while application of a positive voltage signal has the effect of impression of (V−V′). Therefore, response to the state chosen by the negative voltage signal is quickened by the effective voltage increment, while response to the state chosen by the positive voltage signal is slowed by the effective voltage decrement; also as V′ increases, (V−V′) does not reach the threshold, which means no response.
Under the above-said condition that the internal DC voltage component becomes larger, there is even a case that liquid crystal molecules themselves are electrolyzed. Recently, as the stability of liquid crystal materials increases, such electrolysis rarely occurs as far as the drive voltage is within a normal range; however, there still remains the possibility of picture quality deterioration being caused by the effective DC component of drive voltage waveform.
For the above reason, it has been believed that it is a good practice to keep the LCD drive voltage waveform electrically neutral and use an AC drive system in which positive and negative voltages alternate and are symmetrical with respect to 0 V as seen in rectangular waveforms with an offset voltage of 0 V, sinusoidal, cosine and triangular waveforms.
For example, TN mode LCDs use rectangular waveforms with 0 V offset voltage for drive and rectangular waveforms for TFT gate element drive in order to keep the drive voltage waveform electrically neutral.
In SSFLC mode LCDs, the following drive method has been used: when applying a pulse voltage to select either the On state or the Off state, the voltage waveform with the reverse polarity is combined to offset the DC component within one selection time or reverse polarity voltage pulses are inserted so as to offset the DC component on the average over a longer time (e.g. plural frames).
However, in these SSFLC mode LCDs, in order to maintain electrical neutrality, voltage waveforms practically not contributing to liquid crystal drive have to be inserted for as long a time as the state selection voltage waveforms, which necessitates shortening of bit plane time with resultant deterioration in brightness and gradation characteristics. Still further, the time allowed for liquid crystal response is shortened, which increases the load on the liquid crystal material.